The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen" which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the carbonaceous liquid product is called "shale oil".
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes the formation of a fragmented permeable mass of oil shale particles in a subterranean formation containing oil shale by undercutting a portion of the subterranean formation leaving unfragmented formation supported by a plurality of pillars. The pillars are removed, e.g., with explosive, and the unfragmented deposit is expanded to provide a permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying combustion zone feed into the retort on the trailing side of the combustion zone to advance the combustion zone through the fragmented mass. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products and a residual carbonaceous material. The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniform fragmented permeable mass of formation particles having a reasonably uniformly distributed void fraction so gases can flow uniformly through the retort resulting in maximum conversion of kerogen to shale oil. A uniformly distributed void fraction in the direction perpendicular to the direction of advancement of the combustion zone is important to avoid channeling of gas flow in the retort. In preparation for the described retorting process, it is important that the formation be fragmented and displaced, rather than simply fractured, in order to create high permeability; otherwise, too much pressure differential is required to pass gas through the retort.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed leaving the unfragmented portion supported by pillars. The pillars are then explosively expanded and after a time delay the unfragmented formation is explosively expanded, thereby filling the void created by the undercut with a fragmented permeable mass of particles.
To promote uniform void fraction distribution, pillars are explosively expanded first and then, after a time delay, the remaining unfragmented formation is explosively expanded either in a single explosion or in a further series of explosions in a single round.
The general art of blasting rock formations is discussed in The Blasters' Handbook, 15th Edition, published by E. I. duPont de Nemours & Company, Wilmington, Delaware.
U.S. Patent Application Ser. No. 833,240, by Gordon B. French, titled EXPLOSIVE PLACEMENT FOR EXPLOSIVE EXPANSION TOWARD SPACED APART VOIDS, which is assigned to the assignee of the present application, describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids containing support pillars. The pillars are explosively expanded to spread the particles thereof uniformly across the void, and unfragmented formation adjacent the void is explosively expanded toward the void before overlying, unsupported formation can cave into the void. Said U.S. Patent Application Ser. No. 833,240 is incorporated herein by this reference.
Application Ser. No. 929,250 filed July 31, 1978 by Thomas E. Ricketts titled METHOD FOR EXPLOSIVE EXPANSION TOWARD HORIZONTAL FREE FACES FOR FORMING AN IN SITU OIL SHALE RETORT describes the formation of a retort and recovery of liquid and gaseous products from the retort and is incorporated herein by reference.
There are several other patents which describe the recovery of liquid and gaseous products from oil shale which include a discussion regarding the removal of pillars from mined out areas.
U.S. Pat. No. 3,980,339 issued Sept. 14, 1976 to David D. Heald describes forming a substantially horizontal in situ oil shale retort by mining out an area at the base of an oil shale deposit leaving overlying deposit supported by a plurality of pillars. The pillars are removed by drilling a plurality of holes into the pillars for receiving explosive. The holes are shown as being drilled a short distance into all four vertical faces of each rectangular pillar. Explosive is then placed into the holes wherein the type of explosive and sequence of setting off the charges is chosen so as to form rubble of a desired size. There is no disclosure however of a specific sequence or timing to be used in detonating the explosive in the pillars.
U.S. Pat. No. 3,316,020 issued Apr. 25, 1967 to E. V. Bergstrom relates to a process of in situ retorting of oil shale using roof failure methods. Horizontal slots called passageways are mined into the oil shale and cross openings are then drilled between the slots. The cross openings are drilled at an angle other than normal to the vertical wall of the slots, preferably at an angle of 45.degree. to the plane of the vertical wall. The cross openings are used as shotholes, with explosive placed along the length of each shothole to develop a desired amount of force. Explosive is detonated and a portion of the wall is displaced into the adjacent horizontal slot. This causes the roof to cave, thereby creating an in situ oil shale retort. Retorting is then commenced and shale oil products recovered.
U.S. Pat. No. 3,434,757 issued Mar. 25, 1969 to M. Prats describes detonation of explosive in arches between parallel tunnels in oil shale to create a large unsupported roof area that collapses into the tunnels. The explosive is shown as being placed into one smaller "tunnel" drilled into each arch. Additional formation is fragmented by sequential detonation of a series of explosives to form permeable zones in the oil shale, and hot fluid is passed through the permeable zones for producing shale oil.
Although the prior art teaches the removal of pillars from within or between voids which have been mined into oil shale formation, there is a need in the art for a method which includes a detailed process for explosively expanding the pillars. Such a detailed process should include steps which promote uniform distribution of pillar fragments into the void. The uniform distribution of pillar fragments can result in the formation of a fragmented permeable mass of oil shale particles having a substantially uniformly distributed void fraction.